1. Technical Field
The present invention relates to electrode catalysts for oxygen reduction for electrochemical systems which use acidic electrolytes in the fields of water electrolysis, inorganic/organic electrolysis, fuel cells, etc. More particularly, the invention relates to an electrode catalyst for oxygen gas diffusion electrodes, namely, oxygen electrode of fuel cells which use solid polymer electrolyte membranes. An oxygen electrode means a cathode.
2. Description of Related Art
Noble metals, in particular, platinum, are stable over a wide potential range and have high catalytic activity for various reactions, and thus are used as electrode catalysts for various electrochemical systems.
Some metal oxides are stable in acidic electrolytes over a wide potential range. Therefore, research has been conducted on enhancing the catalytic activity of platinum by allowing metal oxides to coexist with platinum (for example, Non-patent Documents 1 and 2, and Patent Document 1). However, the catalytic activity of metal oxides themselves has not been evaluated in detail.
With respect to catalysts having high oxygen reduction activity, other than noble metals, use of spinel-type and perovskite-type oxides (Patent Document 2) and double oxides, such as oxides having a columbite structure, an ilmenite structure, an olivine structure, a NASICON structure, and the like as catalysts for fuel cells, air cells, oxygen sensors, etc. (Patent Document 3) has been known. Furthermore, an electrode catalyst in which rare-earth oxide fine particles are mixed as a promoter with noble metal catalyst fine particles and which is suitable for use in gas diffusion electrodes for brine electrolysis and the like (Patent Document 4) has also been known.
Furthermore, in an apparatus in which an ozone generator for supplying an ozone-containing gas is installed upstream of a fuel cell that uses a solid polymer electrolyte membrane, a method has been known in which as a catalyst suitable for reduction reaction of ozone, a metal oxide selected from WO3, TiO2, ZrO2, PtO, Sb2O4, and Sb2O3 is used as an electrode catalyst (Patent Document 5). In addition, as oxide-based electrode catalysts, use of TiO2, Ta2O5, and the like has been known (Patent Documents 6 to 10).
Patent Document 1: Japanese Unexamined Patent Application Publication No. 9-167620
Patent Document 2: Japanese Unexamined Patent Application Publication No. 7-289903
Patent Document 3: Japanese Unexamined Patent Application Publication No. 2003-200051
Patent Document 4: Japanese Unexamined Patent Application Publication No. 2004-197130
Patent Document 5: Japanese Unexamined Patent Application Publication No. 2004-95263
Patent Document 6: Japanese Unexamined Patent Application Publication No. 3-252057
Patent Document 7: Japanese Patent No. 3149629
Patent Document 8: United States Patent Application No. 2004/0058808
Patent Document 9: PCT Japanese Translation Patent Publication No. 2001-522122
Patent Document 10: Japanese Unexamined Patent Application Publication No. 2005-63677
Non-patent Document 1: J. Shim et al., J. Power Sources, 102, 172 (2001)
Non-patent Document 2: A. Katayama, J. Phys. Chem., 84, 376 (1980)